The present invention relates to spectral control during transmit operations. In particular, a transmit spectrum for medical imaging is controlled using an aperture function.
In medical diagnostic ultrasound, transmit acoustic beams are formed by generating transmit waveforms independently for a plurality of different elements. Relative delays and apodization cause the acoustic energy to generate a transmit beam focused at a point or along a region. Sixty-four, 128, 256 or other number of elements and associated transmit channels are typically used. Each transmit channel consumes power, adds to cost and adds to size. Complex waveform generators, such as used for generating sinusoidal or near sinusoidal waveforms are expensive. Bipolar, unipolar or simpler waveform generators may use less energy, space and cost. However, bipolar and unipolar waveforms limit the ability to control the transmit spectrum. For example, unipolar and bipolar waveforms have more energy at second harmonic frequencies of fundamental transmit frequencies (twice the fundamental) than a sinusoid of a same number of cycles. As a result, information at second harmonic frequencies isolated during receive operation undesirably includes information generating by the transmitter rather than by tissue or reflection.
In “Transmit Aperture Processing for Non-Linear Contrast Agent Imaging” by Krishnan and O'Donnell, an additional phase shift controls the transmitted spectral content. A 90° phase shift is applied to every other element. At fundamental frequencies, the resulting amplitude at the focal region is decreased by approximately the square root of 2. At the second harmonic frequencies, the 90° phase shift is converted to a 180° phase shift. At the focal region, the waveforms acoustically combine. The resulting combination of waveforms with a 180° phase difference cancels or reduces energy due to the transmitters at the second harmonic. However, the phase shift is applied to sinusoidal waveforms.
U.S. Pat. No. 6,193,659 implements spectrum control for transmit operations with more simplistic waveform generators, such as bipolar or unipolar waveform generators. Since these waveform generators may not be able to produce a 90° phase shifted waveform, a delay of one-quarter wave length at the center frequency is used. The one-quarter wave length delay is applied to every other or to groups of transmit elements. The amplitude at the fundamental frequency is decreased by a factor of the square root of two, and information at second harmonic frequency of the center frequency is reduced or canceled. However, the 3 db or other loss at the fundamental center frequency may require larger voltages to be applied to the transducer to achieve the desired transmit power. The gain is higher at lower frequencies than at the center frequency which may introduce a low frequency artifact. The cancellation of the second harmonic is narrow so that wide band harmonic signals may not be well suppressed.